Monday 13 May 2013 03.00 EDT
First published on Monday 13 May 2013 03.00 EDT

Cool anger drives Andrew Adonis's first-hand account of how Labour tried to stop the Liberal Democrats handing Britain over to a reactionary and incompetent Conservative administration. As a Blairite education and transport minister and a former member of the Social Democratic party, Adonis had spent his adult life believing a "progressive coalition" could unite the centre and left of British politics.

His five days in May 2010 negotiating on Labour's behalf disabused him of that notion and much else besides. "Clegg wouldn't put the Tories in power, throwing over a British Liberal tradition going back a century and a half as a progressive anti-Tory party," he thought as the electorate returned a hung parliament. When they heard that David Cameron was making Clegg a generous offer, Gordon Brown and much of the cabinet thought the "process would turn to our favour once the Tories and Lib Dems had rehearsed the extent of their differences".

They were not the only ones who believed the Liberals were a leftwing party. I lost count of and patience with the Billy Bragg types of the past decade, who announced that they were voting Liberal Democrat, and then stood back as if expecting a round of applause. Apparently, to be a truly right-thinking leftwinger, you had to rally behind Nick Clegg, a public-school former Eurocrat, whose ideologues had denounced "soggy socialism and corporatism" in the Lib Dems' Orange Book manifesto.

Adonis goes to some effort to explain that the votes were there if Clegg had been interested in stopping Cameron. The Conservatives had 306 seats, but needed 326 for an overall majority. The Lab–Lib total was 315. The five SDLP, Alliance and independent members from Northern Ireland were left-leaning, as was the one English Green. The Scottish and Welsh nationalists could not side with the Tories because their electorates would have "killed" them, as Gordon Brown explained. Meanwhile Cameron's decision to support their rivals had alienated MPs from Northern Ireland's Democratic Unionist party (DUP). Brown was ready to step down, once the new "progressive" coalition was running, and had given public and private assurances to this effect. His vanity would not get in the way of making it work.

Tony Blair and others warned that if no one had won the 2010 election, Labour had lost it and should accept defeat. Northern MPs, who had dealt with Liberal Democrat councils, added that this was not a party to trust. Nevertheless, if the Liberal Democrats' prime concern had been keeping the Conservatives out, a deal could have been made.

As it was, those northern MPs were prescient.

In a conference call with several witnesses, Gordon Brown had an amiable discussion with Nick Clegg on forming a centre-left government. Paddy Ashdown then briefed the BBC that the conversation "was a diatribe, a rant, and that Gordon Brown was threatening in his approach to Nick Clegg". After another apparently amicable encounter, Ashdown said: "The meeting with you guys was a disaster. The Tories will give us massively more than you guys and they were respectful, whereas your body language was, my guys said, truly shocking."

The Liberals' attempts to form a progressive coalition were a pretence. The party wanted to fob off its leftish activists by telling them that Labour's behaviour had made a deal impossible, and to use sham talks to extract more concessions from the Conservatives.

Adonis's most extraordinary revelation – although less extraordinary in retrospect – was that David Laws told Labour that the Lib Dems agreed with Osborne on austerity. Laws, a former banker, who was later caught robbing the taxpayer as an MP, did not try to moderate Conservative demands in the coalition negotiations. He and Clegg agreed wholeheartedly with them. As Adonis says, the Liberal Democrats did not form an alliance with the Conservatives despite Osborne but because of Osborne. (Adonis is not making a propaganda point, incidentally. Laws's own account of the negotiations bears him out.)

I suppose it is naive of me to add that Liberal Democrats had campaigned against austerity in the election campaign – Clegg warned of riots on the streets. But their promises on the economy, like their promises on tuition fees, were simply for show, and the reader is left wondering if Nick Clegg has ever made a promise he intended to keep.

Adonis wrote 5 Days in May just after the Cameron–Clegg coalition took office in 2010. He put it in a bottom drawer when he took a job with a non-partisan thinktank, and left it there until he returned to working full-time for Labour. Publishing three years on makes this a far more devastating work. For what have the Liberal Democrats achieved? Their belief in austerity has condemned Britain to years of stagnation. They have failed to achieve any significant constitutional reform. They have not stopped Cameron promising an "in-out" referendum on Europe, while in health, education, welfare and home affairs, Conservative politicians and conservative priorities have dominated.

Adonis, the Whitehall insider, says that Clegg bungled the coalition negotiations as he bungled so much else. He did not understand how power in Britain works. Clegg should have demanded significant ministerial posts for himself and his lieutenants, but settled for the vainglorious title of deputy prime minister instead. He then compounded his folly by making constitutional reform the Lib Dem priority. Since constitutional reform has little popular support, it was easy for the Conservatives to subvert. As for being deputy prime minister, if it does not quite live down to the old description of the American vice presidency as "not worth a bucket of warm piss", it is not worth much more.

The behaviour of Liberals in power shows that the only vehicle for progressive politics is the Labour party. It's not much of a vehicle. Its engine is usually choking, its exhaust is usually spewing, its passengers are usually stabbing one another in the back, and its driver is usually heading at full speed in the wrong direction. But as Adonis concludes at the end of this revelatory and quietly shocking book, it's all there is.

This article was amended on Monday 13 May to remove two rogue apostrophes.